Field of the Invention
The invention relates to a method for producing expandable graphite intercalation compounds by reacting crystalline graphite particles having a laminar structure with an intercalation agent which contains a strong protonic acid, an oxidizing agent and a phosphoric acid. The graphite intercalation compounds can be used to produce expanded graphite and to form graphite foils, graphite laminates and graphite seals. The invention also relates to a graphite foil providing an improved fluid tight graphite seal.
Graphite intercalation compounds in which the graphite is a carrier of positive charges and anions are intercalated between layer planes of a crystal lattice of the graphite have become increasingly more significant technically over the last 25 years because of their capacity to expand when heat is supplied. Graphite bisulphates and graphite nitrates in particular are important starting products for producing expanded graphite, or graphite expandate, which is in turn processed further by compression to form molded bodies of a wide variety of types, in particular plates and foils. A plurality of products for sealing technology, such as flat seals, stuffing box packings or spherical cap seals, are produced among other items, from plates and foils of that type, as well as directly from graphite expandate. In addition to optimizing the properties of sealing materials such as resilience behavior, chemical resistance, sliding properties or the prevention of corrosion on the sealing surfaces, because of the continuously increasing demands on sealing materials for reasons of environmental protection and the protection of health and safety standards at work, attempts have been made to minimize the permeability of sealing materials and to increase their strength. The improvement of the oxidation stability is an additional target for high-temperature uses. A key to pursuing those targets lies in the manner of producing the starting product, namely the graphite intercalation compound.
According to known methods for producing sealing materials, good crystalline flake graphite is reacted with a mixture of concentrated sulfuric acid and nitric acid, with fuming nitric acid (U.S. Pat. No. 3,404,061), or with a mixture of hydrogen peroxide and concentrated sulfuric acid (U.S. Pat. No. 4,091,083). Excess acid is removed by washing with water and a graphite salt which is obtained in that way is expanded by rapid heating to temperatures of more than 800xc2x0 C., after a drying step. The expandate, which has a very low bulk weight, is then processed further to form sealing materials or other products. According to another method (U.S. Pat. No. 4,895,713), the process is carried out as anhydrously as possible with such a small amount of intercalation agent that the reaction mixture does not have an excess of liquid and the graphite intercalation compound no longer has to be washed but can instead be expanded directly. A phosphate or phosphoric acid can be added to the reaction mixture in order to improve the oxidation stability of the products produced from the graphite expandate. A disadvantage of the above-mentioned methods is that, despite the use of mainly concentrated acids in the intercalation mixture, generally substantial amounts of water are always present. It is known that salt-type graphite intercalation compounds are hydrolyzed by water. Therefore, if water is present in the reaction mixture, the largest possible intercalation effect cannot be achieved. The structure of the graphite expandate produced from the graphite intercalation compound obtained in that way is admittedly quite good, but is still not optimal. Graphite intercalation compounds produced in accordance with U.S. Pat. No. 3,333,941 do not have an optimal structure either. According to the method which is taught therein, it is possible to work with both very small amounts and very large amounts of intercalation agents (range: 0.25 to 4 g intercalation agent to 1 g graphite). A fundamental characteristic of that method, however, is the addition of phosphorus pentoxide to the intercalation mixture in amounts of 2 to 500 percent by weight with respect to the amount of graphite being used. Washing with water must not take place after the end of the intercalation reaction. An addition of phosphoric acid to the reaction mixture is ineffective. However, it was not the aim of the invention to produce graphite intercalation compounds having a great expanding capacity. The products obtained according to that method are intended for use as mulch in farming, as flame-retardant additives or as pH-value-regulating measures. They could not, therefore be used for the production of high-grade sealing materials. The expansion factor of those graphite intercalation compounds, which is at 1 to a maximum of 200, is accordingly comparatively low. Tests have shown that a serious, practical problem when working according to that method is the handling of the phosphorus pentoxide and the working with that substance. Apart from dusting with the aggressive powder all at once, it cannot be mixed-in homogeneously, or it can only be mixed-in homogeneously with difficulty, particularly when only working with small amounts of intercalation liquid. The mixture heats up and lumps start to form even when mixing phosphorus pentoxide with natural graphite. That happens even if phosphorus pentoxide is mixed with a reaction mixture made up of liquid intercalation agent and natural graphite. Such a coalesced reaction mixture is difficult to handle. An addition of water, which could solve that problem, is counter-productive, because it results in the reaction mixture heating up further and uncontrollably and the actual aim of working as anhydrously as possible is ruined.
It is accordingly an object of the invention to provide a method for producing expandable graphite intercalation compounds using phosphoric acids, and a graphite foil, which overcome the hereinafore-mentioned disadvantages of the heretofore-known methods and products of this general type.
Therefore, it is an object of the invention to develop a method for producing salt-type graphite intercalation compounds. In this method, graphite intercalation compounds can be produced with small amounts of intercalation agents and a substantial exclusion of water, which have a very great expansion capacity and which, moreover, have a phosphorus content that gives the products produced from the graphite intercalation compound through the stage of the graphite expandate a high level of fluid tightness, high oxidation stability and an improved strength property. It is noted that the term xe2x80x9cfluidsxe2x80x9d as used herein refers to both liquid and gaseous media.
Another object of the invention was to achieve this target while using as small an amount of intercalation agent as possible.
Furthermore, it was an object of the invention to make available an expanded graphite which is suitable for producing products such as graphite foils, graphite laminates, graphite gaskets, graphite packings, graphite packing rings and graphite packing yarns. The underlying object of the invention was, furthermore, to develop a graphite seal having excellent fluid tightness.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for producing an expandable graphite intercalation compound, which comprises reacting crystalline graphite particles having a laminar structure, in a temperature range from xe2x88x9210xc2x0 C. to +80xc2x0 C., without an additional supply of water, with a reaction mixture formed of at least one strong concentrated acid, an oxidizing agent for oxidizing the graphite, and a thermal polyphosphoric acid.
All naturally occurring or synthetically obtained grades of graphite that have a laminar or plate-like structure can be used for the production of the graphite intercalation compounds. Sulfuric acid in concentrations from 90% by weight, nitric acid in concentrations from 60% by weight, or phosphoric acid in concentrations from 80% by weight, can be used as the strong acid, the anions of which become bound between the layer planes of the graphite lattice during the intercalation process. Mixtures of the above-mentioned acids can also be used. Preferably, concentrated sulfuric acid is used and particularly preferably red, fuming nitric acid.
All substances known from the prior art can be used as the oxidizing agent. Preferably, concentrated nitric acid, hydrogen peroxide or peroxosulphuric acid are used. These acids are preferably used in a mixture with concentrated sulfuric acid as the intercalation agent. Particularly preferably, red, fuming nitric acid is used. An advantage of this procedure lies in the fact that the red, fuming nitric acid acts, on one hand, as an oxidizing agent and, on the other hand, as an intercalation agent, the anion of which is preferably intercalated between the layer planes of the graphite.
A feature which is essential to the invention is that the reaction mixture contains a thermal polyphosphoric acid. Preferably, thermal polyphosphoric acids having a calculated content of phosphorus pentoxide of greater than or equal to 76% by weight or of greater than or equal to 84% by weight are used. The decision as to which acid concentration is used depends on the respective conditions. Thermal polyphosphoric acids are produced by absorbing phosphorus pentoxide in diluted phosphoric acid. Although their concentration is indicated with the aid of a calculated content of phosphorus pentoxide, they do not contain any phosphorus pentoxide. Instead, depending on the concentration, they contain varying amounts of highly condensed phosphoric acids in a mixture of the general formula H(n+2)PnO(3n+1), called polyphosphoric acids. Such polyphosphoric acids are obtainable in the chemical trade. The use of the polyphosphoric acids in the present invention has the following fundamental advantages: the polyphosphoric acids act hygroscopically and impart to the graphite products produced when using them a comparatively high oxidation stability, a comparatively low permeability to fluids and a comparatively great strength. As a result of their hygroscopic action, they absorb water present in the reaction mixture and/or bind water produced during the course of the reaction. The reaction mixture therefore always has a minimum water content. The hydrolysis of the resulting graphite intercalation compounds is suppressed as a result and optimum intercalation is achieved. The latter is a prerequisite for a good expansion capacity of the graphite intercalation compounds produced in this way and for a good structure of the graphite expandate produced therefrom. It follows from the above that even after the ending of the intercalation reaction, the graphite intercalation compound obtained according to the method may only be isolated, but not brought into contact with water. In contrast to phosphorus pentoxide, the polyphosphoric acids are present in liquid form and can therefore be handled without difficulty, which is of particular significance for carrying out the method on a large scale. Apart from this, they can be added to the graphite, the acids or acid mixtures and the reaction mixture at any time, without a detrimental temperature rise taking place. It is advantageous to add the polyphosphoric acid before the start of the intercalation reaction or before mixing the reactants with the natural graphite.
For reasons of simplification, when necessary, the technical term xe2x80x9cthermal polyphosphoric acidxe2x80x9d or xe2x80x9cpolyphosphoric acidxe2x80x9d is used in the singular in the following and in the claims, although in each case one is concerned with mixtures of a plurality of condensed phosphoric acids.
The amount of polyphosphoric acid being added can vary within wide limits. It lies in the range from 1:0.002 to 1:1 parts by weight, with respect to the sum of the acid intended for intercalation and the oxidizing agent being equal to 1. For most practical cases, however, an admixture ratio of 1:0.005 to 1:0.1 parts by weight is sufficient. According to a preferred variant of the invention, an admixture ratio of 1:0.005 to 1:0.03 parts by weight is used.
The quantity of liquids used for the reaction with the graphite and formed of the intercalation agent, the oxidizing agent and the polyphosphoric acid lies in the range from 10 to 200 parts by weight of liquid to 100 parts by weight of graphite. The decision as to how much liquid is used in the individual case depends on the conditions of the individual case and is decided in each case by the person of skill. In general, however, for reasons of process economy and environmental protection, it is advantageous to work with as little liquid as possible, i.e. in the range from 10 to 50 parts by weight of liquid to 100 parts by weight of graphite. In the latter case, the reaction product can be stored without further intermediate steps or treatment or expanded directly. If, however, it is necessary to work with larger amounts of liquid, the excess of reaction liquid is separated from the intercalation compound after the end of the intercalation reaction according to one of the known methods, and the intercalation compound obtained in this way is stored temporarily or processed further. The graphite intercalation compounds obtained in this way have an excellent expansion capacity. It lies at least at 1 to 200, i.e. a one cm3 intercalation compound obtained by pouring takes on a volume of at least 200 cm3 after the expanding. In most cases, expansion factors of approximately 1 to 300 are achieved. The expanding takes place at the fastest rate by applying high temperatures to the graphite intercalation compound. According to one of the known methods, it can be carried out, for example, by blowing the intercalation compound through the flame of a burner or by directing the intercalation compound through a tube heated to the predetermined temperature. In the case of expanding at a temperature of 500xc2x0 C., only average degrees of expansion are obtained. The expansion potential of the intercalation compound is not fully exhausted in this case. Nevertheless, temperatures in this comparatively low range can be used if graphite expandates with high bulk weights, for example for the production of comparatively thick parts, such as plates, are required, or if foils, plates or laminates produced therefrom are to have a high compressive strength. In the case of correspondingly high compression ratios, products produced in this way also have very low leakage values. Preferably, however, temperatures of at least 800xc2x0 C., particularly preferably temperatures from 1000xc2x0 C. and above, are used during expanding. At these temperatures, the expansion potential of the graphite intercalation compounds is fully exhausted and expandates having a bulk weight of 3 g/l or less are obtained.
According to an advantageous variant of the invention, the liquid phase in the reaction mixture is formed of highly concentrated nitric acid of at least 80% by weight and of thermal polyphosphoric acid. The nitric acid in this case serves both as an oxidizing agent and as an intercalation agent. Preferably in this variant, red, fuming nitric acid is used. This intercalation reaction can only take place satisfactorily in dry media. That is because for the course of the reaction the nitronium ion NO2+ has to be formed from nitric acid, releasing water. The nitronium ion oxidizes the graphite and prepares it for the intercalation of nitrate and phosphate ions (see the publication Carbon, Vol. 16, 1978, p. 269-271). If water is present, this reaction is shifted to the side of the nitric acid and too few nitronium ions are available. It is a fundamental advantage of the invention to have made the thermal polyphosphoric acids available for this reaction as a water-binding agent. The reaction can only be carried out on a large scale as a result of this, because, as explained above, working with phosphorus pentoxide is linked with practical difficulties which are too great and, as our own practical tests and the above-mentioned U.S. Pat. No. 3,333,941 shows, leads to unsatisfactory results. A further great advantage of the use of thermal polyphosphoric acid lies in that it can be mixed with nitric acid substantially without any rise in temperature, in contrast to phosphorus pentoxide. This also has a positive effect on the course of the reaction, because the intercalation compounds which result in the reaction mixture can release part of the intercalated substances again if there is too great a temperature increase, with the result that their expansion capacity is reduced. On the contrary, an addition of phosphorus pentoxide to nitric acid leads to a strong temperature increase, as a result of which, in addition to the above-described negative effect, nitrous gases are released to a considerable extent and can then no longer be used for the intercalation reaction. It is vital that the polyphosphoric acid has a sufficiently large content of condensed phosphoric acids of the general formula H(n+2)PnO(3n+1). These are all able to absorb water and support the intercalation reaction as described above. The latter is particularly significant if a polyphosphoric acid having a calculated content of 84% by weight phosphorus pentoxide is used. When this type of acid is used, should this be possible, it is also possible to work with a comparatively small added amount of thermal polyphosphoric acid.
However, it is also possible to work with thermal polyphosphoric acids which have a lower calculated content of phosphorus pentoxide. The use of such types of acid is sensible if comparatively small amounts of water have to be bound or if it is the aim to add a lot of phosphate to the reaction mixture. When proceeding in this way, it is possibly necessary to work with a larger amount of polyphosphoric acid if it is necessary to ensure that the reaction mixture has a sufficient capacity to absorb water. An example of an acid type of this kind is a thermal polyphosphoric acid having a calculated phosphorus pentoxide content of 76% by weight.
The added amount of thermal polyphosphoric acid can lie in a range from 1 part by weight of nitric acid to 0.002 parts by weight of polyphosphoric acid to 1 part by weight of nitric acid to 1.0 parts by weight of polyphosphoric acid. According to a preferred version, the added amount of thermal polyphosphoric acid lies in the range from 1 part by weight of nitric acid to 0.005 parts by weight of polyphosphoric acid to up to 1 part of nitric acid to 0.25 parts of polyphosphoric acid. According to a further preferred version, the added amount of thermal polyphosphoric acid lies in the range from 1 part by weight of nitric acid to 0.005 parts by weight of polyphosphoric acid to up to 1 part by weight of nitric acid to 0.1 parts by weight of polyphosphoric acid. According to a particularly preferred variant of the invention, there is added to the reaction mixture a mixture of nitric acid and thermal polyphosphoric acid that lies in the range from 1 part by weight of nitric acid to 0.005 parts by weight of polyphosphoric acid to up to 1 part by weight of nitric acid to 0.03 parts by weight of polyphosphoric acid.
The liquid phase which is used for the intercalation reaction and which is basically formed of the nitric acid and the thermal polyphosphoric acid is applied in a mixing ratio which lies in the range from 100 parts by weight of graphite to 10 to 200 parts by weight of liquid phase. According to a variant of the method that is preferred for reasons of process economy and environmental protection, a mixing ratio in the range from 100 parts by weight of graphite to 20 to 50 parts by weight of liquid phase is used. Apart from the substances described herein, constituents of the reaction mixture can also be admixtures which are known per se, such as molybdenum compounds for example, that influence or improve the quality of the end products. If amounts of more than 50 parts by weight of liquid phase are added to the graphite for reacting, it is sensible in most cases to separate the excess liquid through the use of one of the known methods after the end of the reaction, and to subsequently further process the graphite intercalation compound obtained in this way. In most cases, the next processing step is the expanding, which is carried out according to one of the known methods at temperatures of at least 500xc2x0 C. If the expansion capacity of the graphite intercalation compound is to be exhausted fully, it is necessary to work with expansion temperatures of at least 800xc2x0 C. and preferably of at least 1000xc2x0 C. An expanded graphite with a bulk weight of at most 3 g/l is then obtained. In the case of an optimal course of the reaction, the bulk weights are around 2 g/l, despite the phosphate content which remains.
The graphite expandates produced from the graphite intercalation compounds can be processed further to form a plurality of products. These are, among others, basically graphite foils, graphite laminates and graphite sheets. The last three products can, for example, be used for purposes of the generation of heat through the use of the Joulean principle, for the purpose of heat conduction or as a heat shield. Their main usage, however, lies in the production of a very wide variety of sealing materials and above all flat gaskets, packing rings and packing yarns. Materials produced according to the method of the invention, and thus also the seals, have a very good oxidation stability and very low permeability values for fluids. Apart from this, they have good strength values, with it being possible for materials having either improved compressive strength or improved tensile strength to be produced. The other outstanding properties of products produced from expanded graphite, such as resistance to high temperatures or corrosion resistance, are not impaired by the new production method.
With the objects of the invention in view, there is also provided a special variant, in which the reaction mixture contains at least 10 parts by weight of thermal polyphosphoric acid, with respect to 1 part by weight of one of the other above-mentioned acids or an acid mixture which can be produced therefrom, and in which the graphite intercalation compound that is obtained is expanded at a temperature of at least 800xc2x0 C. Sealing foils or sealing laminates which are obtained after compression have a gas permeability according to DIN 28090-1 (preliminary test) for nitrogen, with respect to a weight per unit area of 2000 g/m2, of less than or equal to 0.06 mg/(mxc2x7s). Such a low permeability value is unusual for graphite seals which are not specially impregnated or, for example, reinforced with a metal insert, and until now has not been achieved on a large or commercial scale.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is described herein as embodied in a method for producing expandable graphite intercalation compounds using phosphoric acids and a graphite foil, it is nevertheless not intended to be limited to the details provided, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying examples.